1. Field of the Invention
The present invention relates to a light control device used in an optical communication system or an optical information processing system and a method for manufacturing the same device, and more particularly to a light control device capable of suppressing its operating point shift at the time of a temperature change and a method for manufacturing the same device.
2. Description of the Related Art
Up to now a light control device has a structure added for suppressing variation (DC drift) of a bias point to be a base point for operation.
A light control device controls the phase and the intensity of a wave-guided light usually by means of an elector-optic effect and the like. For this, it generates an Electro-optic effect and the like by applying an electric field to an optical waveguide from an electrode provided on the optical waveguide. A light control device can be considered equivalently as a parallel circuit of a capacitor and a resistor to an electric field inputted. Therefore, a DC component contained in an applied electric field generates a transient response to a stepped input and thereby varies the electric field applied to an optical waveguide. Due to such variation of the electric field the bias point of a light control device varies and the level of an optical waveform outputted varies. This variation is a DC drift. Up to now a DC drift has been suppressed by increasing equivalently a resistance value between the electrodes by means of a composition insulating a low resistance film at plural spots.
In a device of this kind, however, since its substrate has a pyroelectric effect, in case that there is a temperature change even under a non-bias condition, an electric charge is generated in the interface between the surface of the substrate and a buffer layer. This phenomenon is called a non-bias temperature shift (temperature shift). Since a conventional device does not have a measure to suppress such a temperature shift, it has had a problem that the yield rate of manufacture is degraded due to such a temperature shift at the time of manufacturing the devices.
An object of the present invention is to provide a light control device capable of suppressing a temperature shift.
A light control device according to the present invention comprises an optical substrate having a pyroelectric effect, a buffer layer formed on said optical substrate, and a first low resistance film covering said buffer layer.
As a concrete composition, in particular, a light control device of the present invention is provided with two optical wave guides near each other, a buffer layer, a low resistance film, a signal electrode, and two ground electrodes.
The two optical wave guides near each other are formed on a crystal substrate having an Electro-optic effect. The buffer layer is formed on part of the optical wave guides and has a smaller dielectric constant than said crystal substrate. The low resistance film is formed on the buffer layer and part of the substrate, and has a smaller resistance value than any of the buffer layer and the substrate. The signal electrode is formed in the vicinity of each of the two optical wave guides. The ground electrodes are formed opposite to each other with the signal electrode between them. The low resistance film is cut at plural spots in the vicinity of the optical wave guides.
A light control device manufacturing method according to the present invention is a method for manufacturing a light control device comprising an optical substrate having a pyroelectric effect, a buffer layer formed on said optical substrate and a first low resistance film covering said buffer layer, said method comprising a process of forming said buffer layer on said optical substrate and a process of forming said first low resistance film covering said buffer layer.